Photon count detector

Abstract

A photon count detector according to the present disclosure comprises: a detection unit having at least one light detection element capable of single photon detection; and a branch unit that causes inputted light to be transmitted stochastically and branches the light into a first path toward the light detection element and a second path different from the first path.

Description

Photon Number Detector

[0001] The present disclosure relates to photon number detectors.

[0002] In recent years, techniques for counting the number of photons in an optical pulse (hereinafter also referred to as "light") have been attracting attention (see, for example, Non-Patent Document 1). For example, a technique for counting the number of photons (photon count) using a photon number discriminator called a superconducting transition edge sensor (TES) has been proposed.

[0003] Japanese Patent Application Laid-Open No. 2023-061076

[0004] However, there is room for improvement in the above-mentioned conventional techniques. For example, in the above-mentioned conventional techniques, in order to count the number of photons, it takes a long time from detecting one photon until the next photon can be detected, and there are limitations on the installation environment due to strict cooling requirements, etc. Therefore, it is desirable to make it possible to realize a flexible configuration for photon number detection.

[0005] Therefore, the present disclosure proposes a photon number detector that can realize a flexible configuration for photon number detection.

[0006] In order to solve the above problems, one form of photon number detector according to the present disclosure includes a detection unit having at least one photodetection element capable of single photon detection, and a branching unit that stochastically transmits input light and branches it into a first path toward the photodetection element and a second path different from the first path.

[0007] FIG. 1 is a diagram illustrating an example of a first configuration of a photon number detector according to an embodiment. FIG. 2 is a diagram illustrating an example of a branching path by a branching section. FIG. 3 is a flowchart illustrating a method for an introduction section. FIG. 4 is a diagram illustrating an example of a second configuration of a photon number detector according to an embodiment. FIG. 5 is a diagram illustrating an example of a photon number detector according to an embodiment. FIG. 6 is a diagram illustrating an example of a photon number detector according to an embodiment. FIG. 7 is a diagram illustrating an example of a photon number detector according to an embodiment. FIG. 8 is a flowchart illustrating a method for a waveguide section. FIG. 9 is a diagram illustrating an example of a sixth configuration of a photon number detector according to an embodiment. FIG. 10 is a diagram illustrating an example of a seventh configuration of a photon number detector according to an embodiment. FIG. 11 is a flowchart illustrating a method for a photon branching section. FIG. 12 is a diagram illustrating an example of the reflectivity of a branching section. FIG. 13 is a flowchart illustrating a method for a photon detection section. FIG. 14 is a diagram illustrating an example of simulation comparison conditions. FIG. 15 is a diagram illustrating an example of experimental results. FIG. 16 is a diagram illustrating an example of experimental results. FIG. 17 is a diagram illustrating an example of experimental results. FIG. 18 is a conceptual diagram illustrating the probability that the input and output photon numbers match. FIG. 19 is a conceptual diagram illustrating the maximum measurement rate at which the input and output photon numbers match.

[0008] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings.

[0009] One or more embodiments (including examples and modified examples) described below can be implemented independently. However, at least a portion of the embodiments described below may be implemented in appropriate combination with at least a portion of another embodiment. These embodiments may include novel features that are different from one another. Therefore, these embodiments may contribute to solving different purposes or problems and may produce different effects.

[0010] The present disclosure will be described in the following order: 1. Embodiment 1-1. Overview of this embodiment 1-2. First configuration 1-3. Implementation method for introduction section 1-4. Second configuration 1-5. Third configuration 1-6. Fourth configuration 1-7. Fifth configuration 1-8. Implementation method for waveguide section 1-9. Sixth configuration 1-10. Seventh configuration 1-11. Implementation method for photon branching section 1-12. Reflectance of branching section 1-12-1. First example (constant reflectance) 1-12-2. Second example (variable reflectance) 1-13. Implementation method for photon detection section 1-14. Experimental results (simulation results) 1-15. Other 2. Effects of the present disclosure

[0011] <1. Embodiment> <1-1. Overview of the Present Embodiment> First, an overview of the premise, etc. related to the present disclosure will be described, and then a configuration, etc. of the present disclosure will be described. In recent years, with the advancement in performance and price reduction of elements capable of single photon detection (also called "single photon detectors") such as SPADs (Single-Photon Avalanche Diodes) and SSPDs (Super Conducting Single Photon Detectors), the application of high-sensitivity light measurement has progressed in a variety of fields, and various applications using these have been commercialized.

[0012] However, the fact that these single-photon detectors can only detect one photon at a time has become apparent as a factor limiting the development of various applications. For example, if it becomes possible to measure multiple photons arriving at the same time (multiple photon number states), it is expected that the functionality and performance of various applications will be expanded and improved by improving the signal-to-noise (S / N) ratio of the measurement and detecting physical phenomena expressed as photon numbers.

[0013] Applications in which the ability to measure such multiple photon number states is expected to expand functionality and improve performance include those shown in Table 1 below. Table 1 shows examples of applications for photon detectors. Table 1 shows an overview, expectations for photon number detection, and important detector performance indicators for each application. Table 1 shows quantum key distribution, quantum communication, biosensing, high-energy physics, and LiDAR (Light Detection and Ranging) as examples of applications, but is not limited to these, as long as photon detectors are applicable to those applications.

[0014]

[0015] From the perspective of expanding applications, important performance indicators for photon number detectors include the probability that the input and output photon numbers match, the maximum measurement rate at which the input and output photon numbers match, and the operating temperature.

[0016] For example, the probability that the number of input and output photons will match is calculated by the following formula (1), where n is the number of input photons and k is the number of output photons, as shown in Fig. 21. Fig. 21 is a conceptual diagram showing the probability that the number of input and output photons will match.

[0017]

[0018] For example, the maximum measurement rate at which the numbers of input and output photons match is as shown in FIG. 22, where the number of input photons is n, the number of output photons is k, and the maximum measurement rate of the element is R ｍａｘ In this case, it is calculated by the following equation (2): Fig. 22 is a conceptual diagram showing the maximum measurement rate at which the numbers of input and output photons match.

[0019]

[0020] This is because accurate detection of physical phenomena expressed as the number of photons is required, and the more photons that are detected at one time, the better the S / N ratio of the measurement.

[0021] From the above perspective, conventional photon number detectors have problems. One example of a conventional photon number detector is an array detector that receives diffused light, but these detectors have problems such as a low probability that the numbers of input and output photons will match, and low efficiency in measuring multiple photons.

[0022] Another example of a conventional photon number detector is the Transition Edge Sensor (TES), but it has issues such as a low measurement rate (maximum of approximately 1 MHz), strict cooling requirements (approximately 100 mK), sensitivity to radiation noise, and limitations on the installation environment. As such, although there are already several examples of conventional photon number detectors, each has its own issues, and no system has yet been provided that satisfies the important performance indexes to a high degree.

[0023] As described above, conventional photon number detectors require the use of diffused light, have strict cooling requirements, and are subject to restrictions on the installation environment, which places constraints on the configuration of the photon number detector and can make it difficult to realize a photon number detector.

[0024] Therefore, the present disclosure proposes a photon number detector that can realize a flexible configuration for photon number detection.

[0025] <1-2. First Configuration> A first configuration of a photon number detector will be described with reference to Fig. 1. Fig. 1 is a diagram showing an example of the first configuration of a photon number detector according to an embodiment. Fig. 1 is a schematic diagram showing the arrangement of each component in a photon number detector 1. For example, Fig. 1 shows an example of a configuration in which light is introduced from an optical fiber.

[0026] The photon number detector 1 has an introduction section 2, a waveguide section 3, branch sections 4a to 4j, and detection sections 5a and 5b. When the branch sections 4a to 4j, etc. are not particularly distinguished, they are referred to as the branch section 4. When the detection sections 5a and 5b, etc. are not particularly distinguished, they are referred to as the detection section 5. Among the components of the photon number detector 1, the waveguide section 3, branch section 4, and detection section 5 may be collectively referred to as the detection unit 20. For example, in the photon number detector 1, light from the introduction section 2 is introduced into the detection unit 20. In this case, a member or structure for appropriately guiding the light may be provided midway from the introduction section 2 to the branch section 4.

[0027] The introduction section 2 introduces light in a photon number state to be measured into the waveguide section 3. The introduction section 2 introduces light in a photon number state arriving from the outside into the waveguide section 3. For example, the introduction section 2 introduces the detection target TG, which is input light from an optical fiber, into the waveguide section 3. For example, a connector is used for the introduction section 2. In FIG. 1 , for example, input light from an optical fiber (such as cable CB in FIG. 2 ) is introduced into the waveguide section 3 by the introduction section 2, which is a connector with the outside (such as an optical fiber connector). When introducing light from an optical fiber in this way, various connectors that connect to an optical fiber can be used as the introduction section 2, and this point will be described later.

[0028] The waveguide 3 guides the light toward the photodetector element 10, which will be described later. In FIG. 1 , for example, the waveguide 3 is made of a transparent resin or the like that fills the space between the detection units 5a and 5b, and details of the waveguide 3 will be described later. The image of the light guided by the waveguide 3 is shown by the dotted arrows in the waveguide 3. That is, in FIG. 1 , the light in the waveguide 3 is guided from left to right.

[0029] 1, the branching units 4a to 4j are provided corresponding to the respective photodetection elements 10 of the detection unit 5. For example, a reflective film or the like is used for the branching unit 4. The branching unit 4 may also be a beam splitter.

[0030] 1, the detecting unit 5a is the detecting unit 5 that has photodetecting elements 10a to 10e and is arranged below the waveguide unit 3. The detecting unit 5a is used as a first detecting unit that is arranged on one side (the lower side in FIG. 1) in a direction (the up-down direction in FIG. 1) that intersects with one direction (the left-right direction in FIG. 1) corresponding to the guiding of light by the waveguide unit 3.

[0031] 1, the detecting unit 5b is the detecting unit 5 that has the photodetecting elements 10f to 10j and is arranged on the upper side of the waveguide unit 3. In this way, the detecting unit 5b is used as a second detecting unit that is arranged on the other side (the upper side in FIG. 1) in a direction that intersects with the one direction corresponding to the guiding of light by the waveguide unit 3.

[0032] When the photodetecting elements 10a to 10j are not particularly distinguished from one another, they will be referred to as the photodetecting elements 10. The photodetecting elements 10 are elements capable of single-photon detection. For example, the photodetecting elements 10 are pixels. For example, single-photon detectors such as SPADs and SSPDs are used as the photodetecting elements 10.

[0033] 1, the detection unit 5a has a plurality of photodetection elements 10a to 10e arranged along one direction corresponding to the guiding of light by the waveguide unit 3. The detection unit 5b has a plurality of photodetection elements 10f to 10j arranged along one direction corresponding to the guiding of light by the waveguide unit 3.

[0034] 1, branch portion 4a is arranged corresponding to photodetection element 10a, branch portion 4b is arranged corresponding to photodetection element 10b, branch portion 4c is arranged corresponding to photodetection element 10c, branch portion 4d is arranged corresponding to photodetection element 10d, and branch portion 4e is arranged corresponding to photodetection element 10e. Also, branch portion 4f is arranged corresponding to photodetection element 10f, branch portion 4g is arranged corresponding to photodetection element 10g, branch portion 4h is arranged corresponding to photodetection element 10h, branch portion 4i is arranged corresponding to photodetection element 10i, and branch portion 4j is arranged corresponding to photodetection element 10j. In this way, multiple branch portions 4a to 4j are provided corresponding to each of the multiple photodetection elements 10a to 10j.

[0035] For example, branching portion 4a is arranged along the surface (top surface in FIG. 1) into which light from photodetecting element 10a is input. Furthermore, each of branching portions 4b to 4e is arranged along the surface into which light from corresponding photodetecting elements 10b to 10e is input. For example, branching portion 4f is arranged along the surface (bottom surface in FIG. 1) into which light from photodetecting element 10f is input. Furthermore, each of branching portions 4g to 4j is arranged along the surface into which light from corresponding photodetecting elements 10g to 10j is input.

[0036] In this way, each branch 4 is arranged along the surface into which light of the corresponding photodetector element 10 is input. That is, each branch 4 is arranged between the waveguide portion 3 and the detection portion 5. In Fig. 1, each of the branches 4a to 4e is arranged between the waveguide portion 3 and the detection portion 5a, and each of the branches 4f to 4j is arranged between the waveguide portion 3 and the detection portion 5b.

[0037] With the above-described configuration, the photodetector element 10a detects light transmitted through the branching portion 4a. Furthermore, each of the photodetector elements 10b to 10e detects light transmitted through the corresponding branching portion 4b to 4e. Furthermore, the photodetector element 10f detects light transmitted through the branching portion 4f. Furthermore, each of the photodetector elements 10g to 10j detects light transmitted through the corresponding branching portion 4g to 4j. In this way, each photodetector element 10 detects light transmitted through the corresponding branching portion 4.

[0038] The branching section 4 stochastically transmits light input from the waveguide section 3 and branches the light into a first path toward the corresponding photodetector element 10 and a second path that reflects the light back to the waveguide section 3.

[0039] This point will be explained using Fig. 2. Fig. 2 is a diagram showing an example of paths branched by a branching unit. Of the photodetecting element 10 and the branching unit 4, Fig. 2 illustrates only one photodetecting element 10a of the detecting unit 5a and the branching unit 4a corresponding to that photodetecting element 10a. In Fig. 2, the branching unit 4a stochastically transmits the optical IP input from the waveguide unit 3 and branches it into a first path FP toward the photodetecting element 10a and a second path SP different from the first path FP.

[0040] Returning to FIG. 1, the explanation will be continued. The waveguide unit 3 guides the light introduced from the introduction unit 2 toward each of the plurality of branch units 4a to 4j. In FIG. 1, the waveguide unit 3 guides the light branched to the second path by the branch unit 4a (second path SP in FIG. 2) toward the branch unit 4f. The waveguide unit 3 also guides the light branched to the second path by the branch unit 4f toward the branch unit 4b. The waveguide unit 3 also guides the light branched to the second path by the branch unit 4b toward the branch unit 4g.

[0041] The waveguide unit 3 also guides the light branched to the second path by branch unit 4g toward branch unit 4c. The waveguide unit 3 also guides the light branched to the second path by branch unit 4c toward branch unit 4h. The waveguide unit 3 also guides the light branched to the second path by branch unit 4h toward branch unit 4d. The waveguide unit 3 also guides the light branched to the second path by branch unit 4d toward branch unit 4i.

[0042] The waveguide unit 3 also guides the light branched to the second path by branch unit 4i toward branch unit 4e. The waveguide unit 3 also guides the light branched to the second path by branch unit 4e toward branch unit 4j. The waveguide unit 3 also guides the light branched to the second path by branch unit 4j toward a further path. For example, if the photon number detector 1 has more than 10 photodetecting elements 10, the waveguide unit 3 guides the light branched to the second path by branch unit 4j toward a photodetecting element 10 provided further away (on the right side in FIG. 1 ).

[0043] As described above, the waveguide unit 3 guides the light from the introduction unit 2 toward the individual photodetection elements 10 (detectors) that make up the detection unit 5. The light reflected from the branching unit 4 is again input to the branching unit 4 by the waveguide unit 3. The branching unit 4 stochastically transmits the light input from the waveguide unit 3 and inputs it to the photodetection elements 10 (detectors). Light that does not transmit through the branching unit 4 is reflected toward the waveguide unit 3 and input again to the branching unit 4. The detection unit 5 is composed of photodetection elements 10 (detectors) that can detect multiple independent single photons. The detection unit 5 stochastically detects the transmitted light that has passed through the branching unit 4.

[0044] As described above, the photon number detector 1 has a repeating structure of the waveguide section 3, the branching section 4, and the detecting section 5. In the photon number detector 1, light reflected from the branching section 4 is guided toward the detecting section 5 until the light is guided toward all of the photodetecting elements 10 (detectors). For example, the photon number detector 1 can easily realize a configuration capable of detecting the number of photons by arranging multiple sets of branching sections 4 and photodetecting elements 10 along the waveguide section 3. For example, the photon number detector 1 can easily arrange a desired number of sets of branching sections 4 and photodetecting elements 10 depending on the number of photons to be detected, and any configuration can be easily realized. Therefore, the photon number detector 1 can realize a flexible configuration for photon number detection.

[0045] As described above, the photon number detector 1 can stochastically create multiple beams of light in a single-photon state by splitting a photon in a multiple-photon number state multiple times with high reflectivity, and these multiple single photons can be measured by a single-photon detection element.

[0046] The photon number detector 1 is expected to provide the following improvements in important performance indicators. For example, the photon number detector 1 can improve the probability of agreement between the input and output photon counts by suppressing variations in the number of measured photons by not diffusing light. Furthermore, for example, the photon number detector 1 can improve the maximum measurement rate at which the input and output photon counts match by using a single-photon detector (such as a SPAD or SSPD) with a high measurement rate as the photodetector. Furthermore, for example, the photon number detector 1 can use a SPAD that can operate at room temperature as the photodetector, which can remove the cooling requirement required for TES and reduce the operating temperature limit.

[0047] With the above-described configuration, the photon number detector 1 can reduce the possibility of not being able to properly count the number of photons when multiple photons enter the introduction section simultaneously (as one event). Also, the photon number detector 1 can reduce the possibility of missing photons that arrive within the dead time when photons enter the introduction section within a short period of time (as different events).

[0048] The photon number detector 1 may have a minimum configuration as shown in Fig. 2. For example, when the photon number detector 1 detects one photon, it may have a configuration as shown in Fig. 1. For example, the photon number detector 1 may be configured by arranging multiple sets of one branching unit 4 and one photodetecting element 10, such as the branching unit 4a and photodetecting element 10a shown in Fig. 2.

[0049] <1-3. Implementation method for the introduction section> Here, a method for implementing the introduction section will be described with reference to Fig. 3. Fig. 3 is a flowchart showing the method for the introduction section. For example, the introduction section has elements as shown in blocks S11 and S12 in Fig. 3. Each block in the flowchart will be described below.

[0050] (Connector with the outside: Corresponding to Block S11) The introduction path for light from the outside may be an optical fiber or a free section, etc. The following can be used as a means for realizing a connector with the outside to connect these to the photon number detector 1, etc.

[0051] For example, when introducing light from an optical fiber, various connectors for connecting to the optical fiber can be used, such as an FC connector, an SC connector, an ST connector, etc.

[0052] For example, when introducing light from free space, an optical focusing mechanism can be used, such as a lens system or various focusing mirrors.

[0053] (Beam splitter: corresponds to block S12) The light input from the connector may be split by a beam splitter, and light detection may be performed using multiple independent paths. The number of splits is optional. Also, the entrance section may not have a beam splitter. In other words, the entrance section may not need to split the light.

[0054] <1-4. Second Configuration> Note that the photon number detector 1 described above is merely an example, and any configuration can be adopted, without being limited to the photon number detector 1. For example, an element other than a connector may be used in the introduction section 2. A second configuration, which is an example in this regard, will be described with reference to FIG. 4. FIG. 4 is a diagram showing an example of the second configuration of the photon number detector according to the embodiment. For example, FIG. 4 shows an example of a configuration in which light is introduced from free space. Note that explanations of points similar to those described above in the first configuration, etc. will be omitted as appropriate.

[0055] As shown in Fig. 4, a photon number detector 1A, which is an example of the second configuration, includes an introduction section 2A instead of the introduction section 2. The introduction section 2A is a lens system for introducing the detection target TG, which is input light, into the waveguide section 3. In Fig. 4, the lens system functions as a connector for connecting input light from free space to the outside. The introduction section 2A, which is a lens system, has a lens and focuses the input light from free space and introduces it into the waveguide section 3.

[0056] The introduction unit 2A introduces light in a photon number state to be measured into the waveguide unit 3. The introduction unit 2A introduces light in a photon number state arriving from the outside into the waveguide unit 3. For example, the introduction unit 2A introduces the detection target TG, which is input light from an optical fiber, into the waveguide unit 3. In this way, when introducing light from free space, an optical focusing mechanism such as the introduction unit 2A can be used. Note that the introduction unit 2A is not limited to a lens system, and various elements can be used as long as it is possible to introduce light from free space.

[0057] <1-5. Third Configuration> Furthermore, for example, the input light to be detected TG may be split into multiple beams before being introduced into the waveguide portion 3. In this case, multiple detection units 20 may be used. A third configuration, which is an example of this point, will be described with reference to FIG. 5. FIG. 5 is a diagram showing an example of the third configuration of the photon number detector according to the embodiment. For example, FIG. 5 shows an example of a configuration in which a beam splitter is used as a component related to the introduction portion. Note that explanations of the same points as those described above in the first and second configurations will be omitted as appropriate.

[0058] 5, a photon number detector 1B, which is an example of the third configuration, has multiple beam splitters 6a to 6c that split input light, and multiple detection units 20a to 20d. When no particular distinction is made between the beam splitters 6a to 6c, they will be referred to as a beam splitter 6. For example, the beam splitter 6 is an optical element that splits (divides) input light into multiple (e.g., two) beams.

[0059] Furthermore, when there is no need to distinguish between the detection units 20a to 20d, etc., they will be referred to as detection units 20. In FIG. 5, the components of the detection unit 20 are similar to those of the detection unit 20 shown in FIG. 1, and therefore only some of the components are referenced with reference numerals. Each detection unit 20 includes a waveguide section 3, a branch section 4, and a detection section 5. The detection unit 20a includes one waveguide section 3, ten branch sections 4 (corresponding to branch sections 4a to 4j in FIG. 1), and two detection sections 5 (corresponding to detection sections 5a and 5b in FIG. 1), each having five photodetection elements 10 (corresponding to photodetection elements 10a to 10j in FIG. 1). Note that the components of each of the detection units 20a to 20d are similar to those of the detection unit 20 shown in FIG. 1, and therefore detailed description thereof will be omitted.

[0060] In FIG. 5 , a beam splitter 6 is disposed between the introduction unit 2, which is a connector for connecting to the outside, and the detection unit 20. Note that the arrangement shown in FIG. 5 is merely an example, and any arrangement can be adopted. For example, the beam splitter 6 may be disposed before the introduction unit 2, such as a connector. In this case, the photon number detector 1B may be provided with four connectors corresponding to the detection units 20a to 20d, respectively. Note that the beam splitter 6 may be included as a component of the introduction unit 2. In this case, the introduction unit 2 of the photon number detector 1B may be configured to include a connector and a beam splitter.

[0061] 5, beam splitter 6a is disposed in the path of light introduced from an external connector, which is introduction unit 2. For example, light introduced from an external connector, which is introduction unit 2, is split into two paths by beam splitter 6a. Beam splitter 6b and beam splitter 6c are disposed in each of the two paths split by beam splitter 6a.

[0062] In Fig. 5, the beam splitter 6b is disposed on one of the two paths split by the beam splitter 6a (the upper path in Fig. 5). For example, the light on one of the two paths split by the beam splitter 6a is split into two paths by the beam splitter 6b.

[0063] In Fig. 5, the light of one of the two paths split by the beam splitter 6b (the upper path in Fig. 5) is introduced into the detection unit 20a. As a result, the waveguide section 3 of the detection unit 20a guides the light introduced from the beam splitter 6b toward the detection section 5 (photodetection element 10) of the detection unit 20a.

[0064] In Fig. 5, the light of the other path (the lower path in Fig. 5) of the two paths split by the beam splitter 6b is introduced into the detection unit 20b. As a result, the waveguide section 3 of the detection unit 20b guides the light introduced from the beam splitter 6b toward the detection section 5 (photodetection element 10) of the detection unit 20b.

[0065] In Fig. 5, the beam splitter 6c is disposed on the other of the two paths split by the beam splitter 6a (the lower path in Fig. 5). For example, the light on the other of the two paths split by the beam splitter 6a is split into two paths by the beam splitter 6c.

[0066] In Fig. 5, the light of one of the two paths split by the beam splitter 6c (the upper path in Fig. 5) is introduced into the detection unit 20c. As a result, the waveguide section 3 of the detection unit 20c guides the light introduced from the beam splitter 6c toward the detection section 5 (photodetection element 10) of the detection unit 20a.

[0067] In Fig. 5, the light of the other path (the lower path in Fig. 5) of the two paths split by the beam splitter 6c is introduced into the detection unit 20d. As a result, the waveguide section 3 of the detection unit 20d guides the light introduced from the beam splitter 6c toward the detection section 5 (photodetection element 10) of the detection unit 20b.

[0068] As described above, photon number detector 1B includes beam splitter 6 that splits light before it is introduced into waveguide portion 3. Photon number detector 1B has a plurality of detection units 20. A plurality of waveguide portions 3 are provided corresponding to the paths branched by beam splitter 6. A plurality of detection portions 5 are provided corresponding to the plurality of waveguide portions 3.

[0069] With the above-described configuration, the photon number detector 1B may perform photodetection using multiple paths by splitting light input from a connector (such as the introduction section 2) using the beam splitter 6. While Fig. 5 shows an example in which light is split twice by the beam splitters 6a to 6c and photodetection is performed using four paths, the number of paths is not limited to four and may be more or less than four. The branched paths may be independent or may be implemented within the same detector array.

[0070] <1-6. Fourth Configuration> Furthermore, the component that splits the light before it is introduced into the detection unit 20 is not limited to the beam splitter 6, and various other components may be used. A fourth configuration, which is an example of this point, will be described with reference to FIG. 6. FIG. 6 is a diagram showing an example of the fourth configuration of the photon number detector according to the embodiment. For example, FIG. 6 shows an example of a configuration in which a wavelength-dependent beam splitter is used as a component related to the introduction section. Note that explanations of the same points as those described above in the first to third configurations will be omitted as appropriate.

[0071] As shown in FIG. 6 , in the fourth configuration, the photon number detector 1C includes multiple wavelength-dependent beam splitters 7a-7c that split input light according to wavelength, and multiple detection units 20a-20d. When the wavelength-dependent beam splitters 7a-7c are not particularly distinguished from one another, they are referred to as wavelength-dependent beam splitters 7. For example, the wavelength-dependent beam splitter 7 is an optical element that splits (divides) input light into light of different wavelength bands. Furthermore, the optical element for splitting wavelengths is not limited to a beam splitter.

[0072] 6, different types of lines are used to schematically indicate the different wavelengths of light introduced into each detection unit 20. The detection units 20a to 20d in FIG. 6 are similar to the detection units 20a to 20d in FIG. 5, and therefore will not be described in detail.

[0073] In FIG. 6 , a wavelength-dependent beam splitter 7 is disposed between the introduction unit 2, which is a connector (lens system) to the outside, and the detection unit 20. Note that the arrangement shown in FIG. 6 is merely an example, and any arrangement can be adopted. For example, the wavelength-dependent beam splitter 7 may be disposed before the connector. In this case, the photon number detector 1C may have four connectors corresponding to each of the detection units 20a to 20d. Note that the wavelength-dependent beam splitter 7 may be included as a component of the introduction unit 2. In this case, the introduction unit 2 of the photon number detector 1C may be configured to include a connector and a beam splitter. In FIG. 6 , polychromatic or polychromatic mixed input light is introduced by the lens system, which is the introduction unit 2.

[0074] 6, wavelength-dependent beam splitter 7a is disposed in the path of light introduced from the connector with the outside, which is introduction unit 2. For example, light introduced from the connector with the outside, which is introduction unit 2, is split into two paths by wavelength-dependent beam splitter 7a. For example, light introduced from the connector with the outside, which is introduction unit 2, is split by wavelength-dependent beam splitter 7a into a path corresponding to the long wavelength side and a path corresponding to the short wavelength side. Wavelength-dependent beam splitter 7b and wavelength-dependent beam splitter 7c are disposed in each of the two paths split by wavelength-dependent beam splitter 7a.

[0075] In Fig. 6, wavelength-dependent beam splitter 7b is disposed on the path corresponding to the longer wavelength of the two paths split by wavelength-dependent beam splitter 7a (the upper path in Fig. 6). For example, of the two paths split by wavelength-dependent beam splitter 7a, the light on the path corresponding to the longer wavelength is split into two paths by wavelength-dependent beam splitter 7b.

[0076] In Fig. 6, of the two paths split by the wavelength-dependent beam splitter 7b, the light in the path corresponding to the longer wavelength (the upper path in Fig. 6) is introduced into the detection unit 20a. For example, light in the wavelength region corresponding to red among the introduced light is introduced into the detection unit 20a. As a result, the waveguide section 3 of the detection unit 20a guides the light introduced from the wavelength-dependent beam splitter 7b toward the detection section 5 (photodetection element 10) of the detection unit 20a.

[0077] In Fig. 6, of the two paths split by wavelength-dependent beam splitter 7b, the light in the path corresponding to the shorter wavelength side (the lower path in Fig. 6) is introduced into detection unit 20b. For example, of the introduced light, light with a wavelength in the range corresponding to green is introduced into detection unit 20a. As a result, the waveguide section 3 of detection unit 20b guides the light introduced from wavelength-dependent beam splitter 7b toward the detection section 5 (photodetection element 10) of detection unit 20b.

[0078] In Fig. 6, wavelength-dependent beam splitter 7c is disposed on the path corresponding to the shorter wavelength side of the two paths split by wavelength-dependent beam splitter 7a (the lower path in Fig. 6). For example, of the two paths split by wavelength-dependent beam splitter 7a, light on the path corresponding to the shorter wavelength side is split into two paths by wavelength-dependent beam splitter 7c.

[0079] In Fig. 6, of the two paths split by wavelength-dependent beam splitter 7c, the path corresponding to the longer wavelength (the upper path in Fig. 6) is introduced into detection unit 20c. For example, light with a wavelength in the blue region of the introduced light is introduced into detection unit 20a. As a result, the waveguide section 3 of detection unit 20c guides the light introduced from wavelength-dependent beam splitter 7c toward the detection section 5 (photodetector element 10) of detection unit 20a.

[0080] In Fig. 6, of the two paths split by wavelength-dependent beam splitter 7c, the light in the path corresponding to the shorter wavelength (the lower path in Fig. 6) is introduced into detection unit 20d. For example, light in the wavelength range corresponding to violet is introduced into detection unit 20a. As a result, the waveguide section 3 of detection unit 20d guides the light introduced from wavelength-dependent beam splitter 7c toward the detection section 5 (photodetector element 10) of detection unit 20b.

[0081] As described above, the photon number detector 1C includes a wavelength-dependent beam splitter 7 that splits light before it is introduced into the waveguide portion 3. The photon number detector 1C has a plurality of detection units 20. A plurality of waveguide portions 3 are provided corresponding to the paths split by the wavelength-dependent beam splitter 7. A plurality of detection portions 5 are provided corresponding to the plurality of waveguide portions 3.

[0082] With the above-described configuration, the photon number detector 1C is provided with the wavelength-dependent beam splitter 7c, which is a beam splitter with wavelength selectivity, so that the input light can be separated into individual wavelength components and received. The photon number detector 1C can be applied to, for example, multicolor imaging (photon count imaging) in sensing and wavelength division multiplexing in communications.

[0083] <1-7. Fifth Configuration> Furthermore, components other than the beam splitter 6, wavelength-dependent beam splitter 7, etc. may be used as the component that splits the light before it is introduced into the detection unit 20. A fifth configuration, which is an example of this point, will be described with reference to FIG. 7. FIG. 7 is a diagram showing an example of the fifth configuration of the photon number detector according to the embodiment. For example, FIG. 7 shows an example of a configuration in which an optical switch is used as a component related to the introduction section. Note that explanations of the same points as those described above in the first to fourth configurations, etc. will be omitted as appropriate.

[0084] As shown in FIG. 7 , in the fifth configuration, the photon number detector 1D has multiple optical switches 8a-8c that branch input light and multiple detection units 20a-20d. When the optical switches 8a-8c are not particularly distinguished from one another, they are referred to as the optical switch 8. For example, the optical switch 8 is a component that branches (splits) input light into multiple (e.g., two) beams by switching the path. For example, the optical switch 8 branches the path of the input light into two paths by switching the path depending on time.

[0085] The detection units 20a to 20d in FIG. 7 are similar to the detection units 20a to 20d in FIG. 5, and therefore a detailed description thereof will be omitted.

[0086] In FIG. 7, the optical switch 8 is arranged between the introduction section 2, which is a connector for connecting to the outside, and the detection unit 20. Note that the arrangement shown in FIG. 7 is merely an example, and any arrangement can be adopted. For example, the optical switch 8 may be arranged before the connector. In this case, the photon number detector 1D may have four connectors corresponding to each of the detection units 20a to 20d. Note that the optical switch 8 may be included as a component of the introduction section 2. In this case, the introduction section 2 of the photon number detector 1D may be configured to include a connector and an optical switch.

[0087] 7, optical switch 8a is disposed on a path of light introduced from an external connector, which is introduction unit 2. For example, light introduced from an external connector, which is introduction unit 2, is branched into two paths by optical switch 8a. Optical switch 8b and optical switch 8c are disposed on each of the two paths branched by optical switch 8a.

[0088] 7, optical switch 8b is disposed on one of the two paths branched by optical switch 8a (the upper path in FIG. 7). For example, light on one of the two paths branched by optical switch 8a is branched into two paths by optical switch 8b.

[0089] 7, the light of one of the two paths branched by the optical switch 8b (the upper path in FIG. 7) is introduced into the detection unit 20a. As a result, the waveguide portion 3 of the detection unit 20a guides the light introduced from the optical switch 8b toward the detection portion 5 (photodetection element 10) of the detection unit 20a.

[0090] 7, the light of the other path (the lower path in FIG. 7) of the two paths branched by optical switch 8b is introduced into detection unit 20b. As a result, the waveguide portion 3 of detection unit 20b guides the light introduced from optical switch 8b toward the detection portion 5 (photodetection element 10) of detection unit 20b.

[0091] 7, optical switch 8c is disposed on the other of the two paths branched by optical switch 8a (the lower path in FIG. 7). For example, the light on the other of the two paths branched by optical switch 8a is branched into two paths by optical switch 8c.

[0092] 7, the light of one of the two paths branched by the optical switch 8c (the upper path in FIG. 7) is introduced into the detection unit 20c. As a result, the waveguide portion 3 of the detection unit 20c guides the light introduced from the optical switch 8c toward the detection portion 5 (photodetection element 10) of the detection unit 20a.

[0093] 7, the light in the other path (the lower path in FIG. 7) of the two paths branched by optical switch 8c is introduced into detection unit 20d, whereby the waveguide portion 3 of detection unit 20d guides the light introduced from optical switch 8c toward detection portion 5 (photodetection element 10) of detection unit 20b.

[0094] As described above, the photon number detector 1D includes an optical switch 8 that branches light before it is introduced into the waveguide portion 3. The photon number detector 1D has a plurality of detection units 20. A plurality of waveguide portions 3 are provided corresponding to the paths branched by the optical switch 8. A plurality of detection portions 5 are provided corresponding to the plurality of waveguide portions 3.

[0095] With the above-described configuration, the photon number detector 1D detects light by switching the path of the light input from the connector using the optical switch 8 according to time. The number of branchings is arbitrary. While Fig. 7 shows an example in which light is branched twice using the optical switches 8a to 8c to perform light detection on four paths, the number of paths is not limited to four and may be more or less than four. The branched paths may be independent, or may be implemented within the same detector array.

[0096] <1-8. Implementation method for the waveguide section> Fig. 8 is a flowchart showing a method for the waveguide section. Fig. 8 is a flowchart showing a method for the waveguide section. For example, the waveguide section (also called a "waveguide") has elements as shown in blocks S21 and S22 in Fig. 8. Each block in the flowchart will be described below.

[0097] (Waveguide: Corresponding to Block S21) The waveguide guides light in the direction of each photodetector element and introduces light directly into the branching portion. The following are examples of the waveguide portion (waveguide).

[0098] For example, an optical fiber is used for the waveguide portion (waveguide). For example, various light-transmitting materials may be used for the waveguide portion (waveguide). For example, a transparent resin, glass, or the like may be used for the waveguide portion (waveguide). Furthermore, the waveguide portion (waveguide) may be a vacuum, air, or the like. Furthermore, when realizing a photon number detector 1 or the like on an optical circuit, a waveguide chip or the like is used for the waveguide portion (waveguide).

[0099] When it is desired to introduce light in a specific direction depending on the wavelength, a holographic waveguide, a photonic crystal waveguide, etc. are used for the waveguide portion (waveguide). When it is desired to dynamically change the waveguide path, a liquid crystal waveguide, etc. are used. When it is desired to realize special light manipulation using a waveguide, a metamaterial-based waveguide, etc. are used for the waveguide portion (waveguide).

[0100] (Mirror: Corresponding to Block S22) A mirror that totally reflects light may be provided at any position in the waveguide. In this case, the following embodiments are given.

[0101] For example, when the refractive index is locally changed within a waveguide, a change in the refractive index within the waveguide may be formed. Also, when a mirror that reflects light of a specific wavelength is formed, this may be realized by introducing a periodic change in the refractive index into the waveguide. In this case, a technique such as Bragg gratings may be used.

[0102] For example, metamaterials can be used to form a mirror that realizes total reflection at a specific angle within the waveguide. For example, a metallic reflective mirror can be used, which is realized by placing a thin metal layer (e.g., aluminum or gold) at a specific position within the waveguide.

[0103] <1-9. Sixth Configuration> In the first to fifth configurations described above, a configuration in which the opposing detectors 5a, 5b are connected by a waveguide section 3 has been described as an example, but the detectors 5 do not have to be arranged facing each other. A sixth configuration, which is an example of this point, will be described using FIG. 9. FIG. 9 is a diagram showing an example of the sixth configuration of a photon number detector according to an embodiment. For example, FIG. 9 shows an example of a configuration in which a mirror is arranged facing a detector and connected by a waveguide. Note that explanations of points similar to those described above in the first to fifth configurations, etc., will be omitted as appropriate.

[0104] 9 , in the sixth configuration, the photon number detector 1E has an introduction section 2, a waveguide section 3, branch sections 4a to 4e, a detection section 5a, and a reflector 9. Of the components of the photon number detector 1E, the waveguide section 3, branch section 4, detection section 5, and reflector 9 may be referred to as a detection unit 21. For example, in the photon number detector 1E, light from the introduction section 2 is introduced into the detection unit 21.

[0105] Introduction portion 2 of photon number detector 1E is similar to introduction portion 2 of photon number detector 1, and therefore a detailed description thereof will be omitted. In Fig. 9, for example, a transparent resin or the like is used in waveguide portion 3 to fill the space between detection portion 5a and reflection portion 9, and light in waveguide portion 3 is guided from the left side to the right side.

[0106] The branching portions 4a to 4e of the photon number detector 1E are similar to the branching portions 4a to 4e of the photon number detector 1. In FIG. 9, the detecting portion 5a is the detecting portion 5 that has photodetecting elements 10a to 10e and is arranged below the waveguide portion 3. The detecting portion 5a is arranged on one side (the lower side in FIG. 9) in a direction (the up-down direction in FIG. 9) that intersects with one direction (the left-right direction in FIG. 9) corresponding to the guiding of light by the waveguide portion 3. The detecting portion 5a is arranged in a position facing the reflecting portion 9 across the waveguide portion 3. Note that the detecting portion 5a of the photon number detector 1E is similar to the detecting portion 5a of the photon number detector 1, and therefore a detailed description thereof will be omitted.

[0107] The reflecting unit 9 is disposed on the other side (upper side in FIG. 9 ) of the direction intersecting with the direction corresponding to the guiding of light by the waveguide unit 3. The reflecting unit 9 reflects light toward the detecting unit 5. A mirror is used for the reflecting unit 9. For example, the reflecting unit 9 is a member that totally reflects input (incident) light. The reflecting unit 9 is disposed in a position facing the detecting unit 5 a across the waveguide unit 3.

[0108] The waveguide unit 3 of the photon number detector 1E guides the light introduced from the introduction unit 2 toward each of the multiple branch units 4a to 4e. In Fig. 9, the waveguide unit 3 guides the light branched to the second path (second path SP in Fig. 2) by the branch unit 4a toward the reflector 9. The waveguide unit 3 also guides the light reflected by the reflector 9 toward the branch unit 4b. The waveguide unit 3 also guides the light branched to the second path by the branch unit 4b toward the reflector 9.

[0109] The waveguide unit 3 also guides the light reflected by the reflecting unit 9 toward the branching unit 4c. The waveguide unit 3 also guides the light branched to the second path by the branching unit 4c toward the reflecting unit 9. The waveguide unit 3 also guides the light reflected by the reflecting unit 9 toward the branching unit 4d. The waveguide unit 3 also guides the light branched to the second path by the branching unit 4d toward the reflecting unit 9.

[0110] Furthermore, the waveguide unit 3 guides the light reflected by the reflector 9 toward the branching unit 4e. Furthermore, the waveguide unit 3 guides the light branched to the second path by the branching unit 4e toward the reflecting unit 9. Furthermore, the waveguide unit 3 guides the light reflected by the reflecting unit 9 toward a destination. For example, if the photon number detector 1E has more than five photodetecting elements 10, the waveguide unit 3 guides the light reflected by the reflecting unit 9 toward a photodetecting element 10 that is provided before the branching unit 4e (on the right side in FIG. 9 ).

[0111] As described above, in the photon number detector 1E, light reflected from the branching unit 4 and reflected by the reflecting unit 9 is input again to the branching unit 4 by the waveguide unit 3. The branching unit 4 stochastically transmits the light input from the waveguide unit 3 and inputs it to the photodetecting element 10 (detector). Light that does not transmit through the branching unit 4 is reflected back toward the waveguide unit 3, reflected by the reflecting unit 9, and input again to the branching unit 4. The detecting unit 5 is composed of the photodetecting element 10 (detector) capable of detecting multiple independent single photons. The detecting unit 5 stochastically detects the transmitted light that has passed through the branching unit 4.

[0112] As described above, the photon number detector 1E has a repeating structure of a waveguide section 3, a branching section 4, a detecting section 5, and a reflecting section 9. In the photon number detector 1E, light reflected from the branching section 4 and the reflecting section 9 is guided toward the detecting section 5 until the light is guided toward all of the photodetecting elements 10 (detectors). For example, the photon number detector 1E can easily realize a configuration capable of detecting the number of photons by arranging multiple sets of branching sections 4 and photodetecting elements 10 along the waveguide section 3. For example, the photon number detector 1E can easily arrange a desired number of sets of branching sections 4 and photodetecting elements 10 depending on the number of photons to be detected, and any configuration can be easily realized. Therefore, the photon number detector 1E can realize a flexible configuration for photon number detection. Furthermore, these structures may be configured such that the set of the reflecting section 9, branching section 4, and detecting section 5 is upside down at some point, or such that the set of the branching section 4 and detecting section 5 is arranged instead of the reflecting section 9.

[0113] <1-10. Seventh Configuration> Furthermore, for example, the photodetector elements 10 may be arranged two-dimensionally (planarly). An example of this seventh configuration will be described with reference to FIG. 10. FIG. 10 is a diagram showing an example of the seventh configuration of the photon number detector according to the embodiment. For example, FIG. 10 shows an example of a configuration in which light is introduced to each element of the array detector using a mirror. Note that explanations of the same points as those described above in the first to sixth configurations will be omitted as appropriate.

[0114] As shown in FIG. 10 , in the seventh configuration, a photon number detector 1F has an introduction section 2, a waveguide section 30, a plurality of branch sections 4, light guide sections 41 and 42, a detection section 50, and a reflection section 90.

[0115] Introduction section 2 of photon number detector 1F is similar to introduction section 2 of photon number detector 1, and therefore a detailed description thereof will be omitted. In Fig. 9 , for example, a transparent resin or the like is used in waveguide section 30 to fill the space between detection section 50 and reflection section 90, and light in waveguide section 30 is guided within waveguide section 30 so as to be input to branch sections 4 corresponding to each photodetection element 10 in detection section 50.

[0116] The detection unit 50 has a plurality of photodetection elements 10 arranged two-dimensionally along a surface. In Fig. 10 , the detection unit 50 has 25 photodetection elements 10. The plurality of photodetection elements 10 are arranged two-dimensionally (in a plane) along one side (the lower side in Fig. 10 ) of the waveguide unit 30.

[0117] In the photon number detector 1F, light guides 41, 42 are arranged at both ends of the multiple light detection elements 10 in one direction along the surface (the left-right direction in FIG. 10 ). The light guides 41, 42 shift the light in another direction along the surface and reflect it in one direction. Mirrors are used for the light guides 41, 42. For example, the light guides 41, 42 are members that totally reflect input (incident) light. The light guides 41, 42 are arranged in opposing positions across the waveguide 30.

[0118] The multiple branching sections 4 are provided corresponding to the respective photodetecting elements 10 of the detection section 50. In Fig. 10, 25 branching sections 4 are provided corresponding to the 25 photodetecting elements 10. Each branching section 4 is arranged along the surface of the corresponding photodetecting element 10 into which light is input. In other words, each branching section 4 is arranged between the waveguide section 30 and the detection section 50.

[0119] With the above-described configuration, in the photon number detector 1F, light from the introduction section 2 is guided toward the individual photodetection elements 10 (detectors) that make up the detection section 5. For example, in the photon number detector 1F, light from the introduction section 2 is guided from left to right toward the row of five photodetection elements 10 (detectors) in the first row (the first (frontmost) from the front in FIG. 10 ). The light is then shifted by one row in the other direction along the surface (the depth direction in FIG. 10 ) by the light guiding section 41 located at the right end, and is guided from right to left toward the row in the second row (the second from the front in FIG. 10 ).

[0120] The light is then shifted by one row in the other direction along the surface (depth direction in FIG. 10 ) by the light-guiding unit 42 located at the left end, and is guided from left to right toward the third row (third from the front in FIG. 10 ). The light is then shifted by one row in the other direction along the surface (depth direction in FIG. 10 ) by the light-guiding unit 41 located at the right end, and is guided from right to left toward the fourth row (fourth from the front in FIG. 10 ). The light is then shifted by one row in the other direction along the surface (depth direction in FIG. 10 ) by the light-guiding unit 42 located at the left end, and is guided from left to right toward the fifth row (fifth from the front in FIG. 10 ).

[0121] As described above, the photon number detector 1F has a repeating structure of the waveguide 30, the branching unit 4, the light guiding units 41 and 42, the detecting unit 50, and the reflecting unit 90. In the photon number detector 1F, light reflected from the branching unit 4, the light guiding units 41 and 42, and the reflecting unit 90 is guided toward the detecting unit 50 until the light is guided toward all of the photodetecting elements 10 (detectors). For example, the photon number detector 1F can easily realize a configuration capable of detecting the number of photons by arranging multiple sets of branching units 4 and photodetecting elements 10 along the waveguide 30. For example, the photon number detector 1F can easily arrange a desired number of sets of branching units 4 and photodetecting elements 10 depending on the number of photons to be detected, and any configuration can be easily realized. Therefore, the photon number detector 1F can realize a flexible configuration for photon number detection.

[0122] <1-11. Implementation Method of Photon Splitting Unit> Here, a method of implementing a photon splitting unit will be described with reference to Fig. 11. Fig. 11 is a flowchart showing the method of the photon splitting unit. For example, the photon splitting unit (also called "splitting unit") has elements as shown in block S31 in Fig. 11. Each block in the flowchart will be described below.

[0123] (Beam splitter: corresponds to block S31) A beam splitter, which is an example of a branching unit, stochastically transmits light input from the waveguide unit and inputs it to the detection unit. Light that does not transmit is reflected back toward the waveguide unit and is input again from the waveguide unit to the branching unit. The beam splitter may have different reflectances corresponding to the individual photodetection elements. Examples of the branching unit include the following.

[0124] For example, the branching section may be formed integrally with the waveguide structure. The branching section may be configured with a highly reflective wall surface required for forming the waveguide as a beam splitter. In this case, the reflectivity of the branching section may be adjusted by adjusting the refractive index of the wall surface.

[0125] For example, the branching section may be a component that reflects light by utilizing the periodic changes in refractive index of Bragg gratings. Alternatively, the branching section may be a multi-mode interference (MMI). For example, an MMI uses a wide waveguide section to excite multiple modes, which then interfere to obtain a specific branching ratio. By changing the shape and size of the interference region, a branching section (such as a beam splitter) with a different reflectivity can be formed.

[0126] For example, the branching unit may use liquid crystal adjustment or the like. In this case, the photon number detector 1 or the like can electrically adjust the refractive index of a specific region in the waveguide using liquid crystal. This allows the photon number detector 1 or the like to dynamically control branching units (beam splitters or the like) with different reflectances in the same waveguide.

[0127] For example, a microring resonator or the like may be used as the branching section. In this case, the photon number detector 1 or the like can couple light at a specific wavelength by arranging a tiny ring structure near the waveguide. This allows the photon number detector 1 or the like to form branching sections (beam splitters or the like) with different reflectivities by using microring resonators with different sizes and coupling strengths.

[0128] <1-12. Reflectance of Branching Section> The reflectance of the branching section 4 can be set arbitrarily. Below, several examples of the reflectance of the branching section 4 are shown. Note that, of the configurations described above, the photon number detector 1E having the sixth configuration is shown as an example below, but the setting of the reflectance of the branching section 4 may also be similarly applied to the photon number detectors 1, 1A to 1D, 1F, etc.

[0129] <1-12-1. First Example (Constant Reflectance)> For example, the reflectance of the branching portion 4 may be constant. An example of this point will be described as a first example using FIG. 12. FIG. 12 is a diagram showing an example of the reflectance of the branching portion. As a specific example, FIG. 12 shows an example where the reflectance of the branching portion is constant. The percentage values ​​shown superimposed on each of the branching portions 4a to 4e in FIG. 12 indicate the reflectance of each of the branching portions 4a to 4e.

[0130] The photon number detector 1E in FIG. 12 shows a case where the branching unit 4 is configured using a beam splitter or the like with a constant reflectance. In the photon number detector 1E in FIG. 12, the multiple branching units 4 have the same reflectance. FIG. 12 shows a case where the reflectance of all five branching units 4a to 4e is 99%. For example, beam splitters with a reflectance of 99% are used for the branching units 4a to 4e. In this way, the reflectance of the branching units 4 may be set to be the same.

[0131] <1-12-2. Second Example (Reflectance Uncertain)> The reflectance of the branching portion 4 may also be different. An example of this point will be described as a second example using FIG. 13. FIG. 13 is a diagram showing an example of the reflectance of the branching portion. As a specific example, FIG. 13 shows an example where the reflectance of the branching portion is uncertain. The percentage values ​​shown superimposed on each of the branching portions 4a to 4e in FIG. 13 indicate the reflectance of each of the branching portions 4a to 4e.

[0132] 13 shows a photon number detector 1E in which the branching section 4 is configured by a beam splitter or the like having a different reflectance for each photodetecting element 10 (detector). In the photon number detector 1E in FIG. 13, at least some of the multiple branching sections 4 have different reflectances.

[0133] In Fig. 13, the branching unit 4a is set to 99%. For example, a beam splitter with a reflectance of 99% is used for the branching unit 4a. The branching unit 4b is set to 80%. For example, a beam splitter with a reflectance of 80% is used for the branching unit 4b.

[0134] In FIG. 13 , the branching unit 4c is set to 60%. For example, a beam splitter with a reflectance of 60% is used for the branching unit 4c. The branching unit 4d is set to 30%. For example, a beam splitter with a reflectance of 10% is used for the branching unit 4d. The branching unit 4d is set to 30%. For example, a beam splitter with a reflectance of 10% is used for the branching unit 4d.

[0135] 13, each of the plurality of branching sections 4 has a reflectance equal to or lower than the reflectance of the branching section 4 that reflected the input light. In the photon number detector 1E of Fig. 13, each of the plurality of branching sections 4 has a reflectance that attenuates with increasing distance from the introduction section 2. In this way, the reflectance of the branching sections 4 may be set to different values.

[0136] <1-13. Implementation Method of the Photon Detector> Here, a method of implementing the photon detector will be described with reference to Fig. 14. Fig. 14 is a flowchart showing the method of the photon detector. For example, the photon detector (also called the "photodetector") has elements as shown in block S41 in Fig. 14. Each block in the flowchart will be described below.

[0137] (Photodetector: Corresponding to Block S41) The detection unit (photodetector) is composed of a plurality of independent photodetection elements, and detects light transmitted through the branching unit (optical branching unit). The photodetection elements may be independent detectors for each element, or may form an array of elements.

[0138] The photodetector element used is capable of detecting a single photon. Examples of the photodetector element include the following. For example, a SPAD or the like may be used as the photodetector element. In this case, the photodetector element detects a single photon by utilizing the electron avalanche that occurs when a reverse bias voltage is high.

[0139] For example, photomultiplier tubes (PMTs) or the like may be used as the photodetector. In this case, the photodetector utilizes the phenomenon that when a photon strikes an electrode (photocathode), thermal electrons are emitted. These electrons repeatedly collide with the electrode in a series, amplifying the number of electrons emitted, and ultimately generating a signal pulse that can be considered as photodetection.

[0140] For example, the photodetector may be an SSPD, etc. In this case, the photodetector uses a superconducting nanowire to detect the localized destruction of the superconducting state caused by the absorption of a photon.

[0141] For example, a photon number detector or the like may be used as the photodetector. In this case, the photodetector itself may be a photodetector having the ability to detect the number of photons (such as a TES, a Microwave Kinetic Inductance Detector (MKID), or a Visible Light Photon Counter (VLPC)). In this case, the detection unit may have one photodetector having the ability to detect the number of photons.

[0142] <1-14. Experimental Results (Simulation Results)> From here, experimental results (simulation results) using the above-described configuration will be shown. For example, the results of verifying the effects when using the configuration of the photon number detector 1E through simulation will be shown. First, the conditions underlying the simulation will be explained. FIG. 15 is a diagram showing an example of simulation comparison conditions. The present method in the second row of the table in FIG. 15 corresponds to the case where the configuration of the photon number detector 1E is used. Furthermore, as shown in FIG. 15, as reference conditions corresponding to the present method, an array detector that receives diffused light in the third row of the table in FIG. 15 and a TES (superconducting transition edge sensor) in the fourth row of the table in FIG. 15 were used as comparison objects.

[0143] Next, the experimental results will be explained using Figs. 16 to 20. Figs. 16 to 20 are diagrams showing examples of experimental results. First, the results of Figs. 16 and 17 will be explained. Fig. 16 shows the probability (correct rate) that the number of input and output photons matches. Result RS1 in Fig. 16 is a graph with the vertical axis representing probability and the horizontal axis representing the number of input and output photons. Line L11 in result RS1 shows the probability in the case of this method. Line L12 in result RS1 shows the probability in the case of an array detector. Line L13 in result RS1 shows the probability in the case of TES.

[0144] FIG. 17 shows the maximum rate of measurement events where the number of input and output photons matches. Result RS2 in FIG. 17 is a graph with the measurement rate on the vertical axis and the number of input and output photons on the horizontal axis. Line L21 in result RS2 shows the measurement rate in the case of this method. Line L22 in result RS2 shows the measurement rate in the case of an array detector. Line L23 in result RS2 shows the measurement rate in the case of TES. As shown in FIGS. 16 and 17, this method is expected to have the best performance in both indicators.

[0145] Next, the experimental results (simulation results) of FIGS. 18 to 20 will be described. First, FIG. 18 shows the beam splitter reflectivity coupled to each element of the photodetector. Setting RS3 in FIG. 18 is a graph with reflectivity on the vertical axis and the number of photodetecting elements 10 (branching sections 4) on the horizontal axis. For example, the smaller the number on the horizontal axis shown in setting RS3, the closer the photodetecting elements 10 (branching sections 4) are to the introduction section 2. For example, a branching section 4 for which the value on the horizontal axis shown in setting RS3 corresponds to 0 corresponds to the reflectivity of the branching section 4 (branching section 4a in the photon number detector 1E in FIG. 9) corresponding to the photodetecting element 10 closest to the introduction section 2.

[0146] Line L31 in setting RS3 indicates the reflectance of the branching section 4 corresponding to each photodetector element 10 when the reflectance R is exponentially decayed. The reflectance R of the branching section 4 corresponding to each photodetector element 10 shown on line L31 is calculated by the following formula (3). For example, line L31 indicates a case where the branching section 4 is configured using a beam splitter or the like with a different reflectance for each detector. Hereinafter, the result when the reflectance is exponentially decayed as shown on line L31 will be referred to as the "first result."

[0147]

[0148] For example, R in formula (3) ０ is 99%, α is a predetermined coefficient, and x ０ represents the case where the reflectance R of the branching portion 4 is 144. Note that the formula (3) is merely an example, and when the reflectance R of the branching portion 4 is to be attenuated, the reflectance R of the branching portion 4 may be calculated using any function.

[0149] Furthermore, line L32 in setting RS3 indicates the reflectance of the branching section 4 corresponding to each photodetector element 10 when set to a constant reflectance. The reflectance R of the branching section 4 corresponding to each photodetector element 10 indicated by line L32 is set to 97%. For example, line L32 indicates a case where the branching section 4 is configured using a beam splitter or the like with a constant reflectance (97%). Hereinafter, the result when the reflectance is constant as indicated by line L32 will be referred to as the "second result."

[0150] Next, the results of Figures 19 and 20 will be described. Figure 19 shows the probability (correct rate) that the number of input and output photons matches depending on the optical branching unit (branching unit 4). Result RS4 of Figure 19 is a graph with the vertical axis representing probability and the horizontal axis representing the number of input and output photons. Line L41 in result RS4 shows the result (first result) when the reflectance of branching unit 4 is different. Line L42 in result RS4 shows the result (second result) when the reflectance of branching unit 4 is constant (97%).

[0151] FIG. 20 shows the maximum rate of measurement events where the number of input and output photons matches depending on the optical branching unit (branching unit 4). Result RS5 in FIG. 20 is a graph with the measurement rate on the vertical axis and the number of input and output photons on the horizontal axis. Line L51 in result RS5 shows the result (first result) when the reflectivity of branching unit 4 is different. Line L52 in result RS5 shows the result (second result) when the reflectivity of branching unit 4 is constant (97%). As shown in FIGS. 19 and 20, it was confirmed that performance was further improved in both indices when the reflectivity R was exponentially decayed.

[0152] <1-15. Others> For example, an information processing device such as a control device that controls the above-described photon number detectors 1 to 1F, etc., or that collects (acquires) information detected (measured) by the photon number detectors 1 to 1F, etc., may be realized by a dedicated computer system or a general-purpose computer system.

[0153] For example, a program for executing the above-described operations is stored in a computer-readable recording medium such as an optical disk, a semiconductor memory, a magnetic tape, or a flexible disk and distributed. Then, for example, the program is installed in a computer and the above-described processing is executed to configure a control device. In this case, the control device may be an external device (e.g., a personal computer) such as the photon number detector 1. Alternatively, the control device may be an internal device (e.g., a processor) such as the photon number detector 1.

[0154] The program may also be stored in a disk device provided on a server on a network such as the Internet, and may be downloaded to a computer. The above-described functions may also be realized by cooperation between an operating system (OS) and application software. In this case, the parts other than the OS may be stored on a medium and distributed, or the parts other than the OS may be stored on a server and may be downloaded to a computer.

[0155] Furthermore, among the processes described in the above embodiments, all or part of the processes described as being performed automatically can be performed manually, or all or part of the processes described as being performed manually can be performed automatically using a known method. In addition, the information including the processing procedures, specific names, various data, and parameters shown in the above documents and drawings can be changed as desired unless otherwise specified. For example, the various information shown in each drawing is not limited to the information shown in the drawings.

[0156] Furthermore, the components of each device shown in the figure are conceptual functional units and do not necessarily have to be physically configured as shown. In other words, the specific form of distribution and integration of each device is not limited to that shown in the figure, and all or part of the devices can be functionally or physically distributed and integrated in any unit depending on various loads, usage conditions, etc. Note that this distribution and integration configuration may also be performed dynamically.

[0157] The above-described embodiments can be combined as appropriate within the scope of the present invention without causing any inconsistency in the processing content. The order of the steps shown in the flowcharts of the above-described embodiments can be changed as appropriate.

[0158] Furthermore, for example, the present embodiment can also be implemented as any configuration that constitutes an apparatus or system, such as a processor as a system LSI (Large Scale Integration), a module using multiple processors, a unit using multiple modules, a set in which other functions are added to a unit, or the like (i.e., a configuration of a part of an apparatus).

[0159] In this embodiment, a system refers to a collection of multiple components (devices, modules (components), etc.), regardless of whether all of the components are in the same housing. Therefore, multiple devices housed in separate housings and connected via a network, and a single device in which multiple modules are housed in a single housing, are both systems.

[0160] Furthermore, for example, this embodiment can have a cloud computing configuration in which one function is shared and processed jointly by a plurality of devices via a network.

[0161] For example, an information processing device such as the control device described above may be realized by a computer having the following configuration. The computer includes a CPU, RAM, ROM (Read Only Memory), HDD (Hard Disk Drive), a communication interface, and an input / output interface. Each part of the computer is connected by a bus.

[0162] The CPU operates based on programs stored in the ROM or HDD and controls each part. For example, the CPU loads the programs stored in the ROM or HDD into the RAM and executes processes corresponding to the various programs.

[0163] The ROM stores boot programs such as a Basic Input Output System (BIOS) that is executed by the CPU when the computer starts up, as well as programs that depend on the computer's hardware.

[0164] The HDD is a computer-readable recording medium that non-temporarily records programs executed by the CPU, data used by such programs, etc. Specifically, the HDD is a recording medium that records information processing programs such as control programs for controlling the photon number detectors 1, 1A-1F, etc. according to the present disclosure, which are examples of program data.

[0165] A communication interface is an interface that allows a computer to connect to an external network (e.g., the Internet). For example, a CPU receives data from other devices and transmits data generated by the CPU to other devices via the communication interface.

[0166] An input / output interface is an interface for connecting an input / output device to a computer. For example, a CPU receives data from an input device such as a keyboard or a mouse via the input / output interface. The CPU also transmits data to an output device such as a display, a speaker, or a printer via the input / output interface. The input / output interface may also function as a media interface for reading programs and the like recorded on a predetermined recording medium. Examples of media include optical recording media such as DVDs (Digital Versatile Discs) and PDs (Phase Change Rewritable Disks), magneto-optical recording media such as MOs (Magneto-Optical Disks), tape media, magnetic recording media, and semiconductor memories.

[0167] For example, when a computer functions as a control device in a photon number detector 1, 1A-1F, etc., the CPU of the computer executes an information processing program loaded onto RAM to realize the functions of the control device, etc. The information processing program according to the present disclosure and data in the storage unit of the control device are stored in the HDD. The CPU reads and executes program data from the HDD, but as another example, the CPU may obtain these programs from another device via an external network.

[0168] Although the embodiments of the present disclosure have been described above, the technical scope of the present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the gist of the present disclosure. Furthermore, components of different embodiments and modifications may be combined as appropriate.

[0169] Furthermore, the effects of each embodiment described in this specification are merely examples and are not intended to be limiting, and other effects may also be obtained.

[0170] 2. Effects of the Present Disclosure As described above, a photon number detector according to the present disclosure (photon number detector 1 in the embodiments, etc.; the same applies hereinafter) includes a detection unit (detection unit 5 in the embodiments, etc.; the same applies hereinafter) that has at least one photodetection element (photodetection element 10 in the embodiments, etc.; the same applies hereinafter) capable of single photon detection, and a branching unit that stochastically transmits input light and branches it into a first path (first path FP in the embodiments, etc.; the same applies hereinafter) heading toward the photodetection element, and a second path (second path SP in the embodiments, etc.; the same applies hereinafter) that is different from the first path.

[0171] In this way, the photon number detector according to the present disclosure probabilistically transmits input light and branches it into a first path toward the photodetector element and a second path different from the first path, thereby adjusting the probability that each photon will be detected by the photodetector element. For example, by reducing the probability of branching to the first path, the photon number detector can estimate that when one photodetector element detects a single photon, the detection is due to a single photon. In this way, the photon number detector can adjust the detection by one photodetector element to a single photon. Therefore, the photon number detector can realize a flexible configuration for photon number detection.

[0172] In the photon number detector according to the present disclosure, the branching portion is a reflective film, and the photodetecting elements are pixels.

[0173] As a result, the photon number detector according to the present disclosure uses the reflective film, which is a branching section, to stochastically transmit input light and branch it into a first path toward a pixel, which is a photodetection element, and a second path different from the first path, thereby making it possible to adjust the probability that each photon will be detected by a pixel. Therefore, the photon number detector can realize a flexible configuration for photon number detection.

[0174] In the photon number detector according to the present disclosure, the branching section is disposed along the surface of the photodetecting element to which light is input, and the photodetecting element detects light transmitted through the branching section.

[0175] As a result, the photon number detector according to the present disclosure stochastically transmits input light through the branching sections arranged along the light-input surface of the photodetector elements, and detects the light transmitted through the branching sections, thereby enabling a combination of branching sections and photodetector elements to be arranged side by side, thereby enabling a flexible configuration for photon number detection to be realized.

[0176] Moreover, the photon number detector according to the present disclosure includes a waveguide section (waveguide section 3 in the embodiment, etc.; the same applies below) that guides light toward the photodetector element, and an introduction section (introduction sections 2, 2A in the embodiment, etc.; the same applies below) that introduces light in a photon number state to be measured into the waveguide section, and the branching section stochastically transmits the light input from the waveguide section and branches it into a first path and a second path that reflects the light toward the waveguide section.

[0177] As a result, the photon number detector according to the present disclosure can appropriately guide the light by guiding the introduced light toward the photodetector element, probabilistically transmitting the guided light, and branching it into a first path and a second path that reflects the light toward the waveguide section.

[0178] Furthermore, in the photon number detector according to the present disclosure, the detection section has a plurality of photodetection elements, a plurality of branch sections are provided corresponding to the plurality of photodetection elements, and the waveguide section guides the light introduced from the introduction section toward each of the plurality of branch sections.

[0179] As a result, the photon number detector according to the present disclosure can stochastically transmit input light and appropriately detect multiple photons by using multiple photodetecting elements. For example, by reducing the probability of each photodetecting element being branched to the first path, the photon number detector can estimate that when one photodetecting element detects a single photon, the detection is due to a single photon. As a result, the photon number detector can estimate the number of photodetecting elements detected as the number of photons. Therefore, the photon number detector can realize a flexible configuration for photon number detection.

[0180] In the photon number detector according to the present disclosure, the detection section has a plurality of photodetection elements arranged along one direction corresponding to the guiding of light by the waveguide section.

[0181] As a result, the photon number detector according to the present disclosure can appropriately arrange the photodetector elements in accordance with the waveguiding by arranging the photodetector elements along one direction corresponding to the waveguiding of light, thereby enabling the photon number detector to realize a flexible configuration for photon number detection.

[0182] In the photon number detector according to the present disclosure, the plurality of branching sections are disposed between the waveguide section and the detecting section.

[0183] As a result, the photon number detector according to the present disclosure can appropriately arrange the branching sections in accordance with the waveguiding by arranging the branching sections between the waveguide section and the detector section, thereby enabling the photon number detector to realize a flexible configuration for photon number detection.

[0184] In addition, the photon number detector according to the present disclosure includes a first detection unit which is a detection unit arranged on one side of a direction intersecting with one direction, and a second detection unit which is a detection unit arranged on the other side of the direction intersecting with the one direction.

[0185] As a result, the photon number detector according to the present disclosure can efficiently arrange the detecting units by arranging the detecting units on both sides of a line intersecting a direction corresponding to the waveguiding of light (facing arrangement), thereby enabling the photon number detector to realize a flexible configuration for photon number detection.

[0186] In addition, in the photon number detector according to the present disclosure, the detection unit is disposed on one side in a direction intersecting one direction.

[0187] As a result, the photon number detector according to the present disclosure can appropriately position the detecting unit in accordance with the waveguiding by arranging the detecting unit on one side in a direction intersecting one direction, thereby enabling the photon number detector to realize a flexible configuration for photon number detection.

[0188] The photon number detector according to the present disclosure also includes a reflector (in the embodiments, reflector 9, 90, etc.; the same applies below) that is arranged on the other side of the direction intersecting the one direction and reflects light toward the detector.

[0189] As a result, the photon number detector according to the present disclosure can appropriately position the reflector in accordance with the waveguiding by arranging the reflector on the other side of a direction intersecting one direction and reflecting light toward the detector, thereby enabling the photon number detector to realize a flexible configuration for photon number detection.

[0190] In the photon number detector according to the present disclosure, the plurality of photodetecting elements are arranged two-dimensionally along a plane.

[0191] This allows the photon number detector according to the present disclosure to efficiently arrange a plurality of photodetecting elements, thereby enabling the photon number detector to realize a flexible configuration for photon number detection.

[0192] In addition, the photon number detector according to the present disclosure is provided with light guide sections (light guide sections 41, 42, etc. in the embodiments; the same applies below) at both ends of the multiple light detection elements in one direction along the surface, which shift the light in another direction along the surface and reflect it in one direction.

[0193] As a result, the photon number detector according to the present disclosure can appropriately guide light to each of a plurality of two-dimensionally arranged photodetector elements by shifting the light in another direction along the surface and reflecting it in one direction, thereby enabling the photon number detector to realize a flexible configuration for photon number detection.

[0194] In the photon number detector according to the present disclosure, the multiple branches have the same reflectance.

[0195] As a result, the photon number detector according to the present disclosure can adjust the detection probability of each photodetector by using multiple branching sections with the same reflectivity, thereby enabling the photon number detector to realize a flexible configuration for photon number detection.

[0196] In the photon number detector according to the present disclosure, at least some of the multiple branching sections have different reflectivities.

[0197] As a result, the photon number detector according to the present disclosure can individually adjust the detection probability of each photodetector by using multiple branching sections, at least some of which have different reflectivities, thereby enabling the photon number detector to realize a flexible configuration for photon number detection.

[0198] In the photon number detector according to the present disclosure, each of the plurality of branching sections has a reflectance equal to or lower than the reflectance of the branching section that reflected the input light.

[0199] As a result, the photon number detector according to the present disclosure can reduce the reflectance closer to the end by setting the reflectance of each branching section equal to or less than the reflectance of the branching section on the light input side, which can increase the probability of detecting photons in, for example, the photon number detector near the end, and can reduce photon detection omissions.

[0200] In the photon number detector according to the present disclosure, each of the plurality of branch sections has a reflectance that decreases with increasing distance from the introduction section.

[0201] As a result, the photon number detector according to the present disclosure can reduce the reflectance of each branch as it moves away from the introduction section, thereby increasing the likelihood of detecting photons in the photodetector element closer to the end, for example, and suppressing missed photon detection.

[0202] Furthermore, the photon number detector according to the present disclosure includes a photon branching section (in the embodiments, a beam splitter 6, a wavelength-dependent beam splitter 7, an optical switch 8, etc.; the same applies below) that branches light before it is introduced into a waveguide section, and a plurality of waveguide sections are provided corresponding to the paths branched by the photon branching section, and a plurality of detection sections are provided corresponding to the plurality of waveguide sections.

[0203] As a result, the photon number detector according to the present disclosure uses a photon branching unit that branches light before it is introduced into the waveguide unit, allowing multiple detecting units to be arranged corresponding to the branches at the photon branching unit, thereby enabling the detecting units to be arranged efficiently. Therefore, the photon number detector can realize a flexible configuration for photon number detection.

[0204] The present technology can also be configured as follows. (1) A photon number detector comprising: a detection unit having at least one photodetection element capable of single photon detection; and a branching unit that stochastically transmits input light and branches it into a first path toward the photodetection element and a second path different from the first path. (2) The photon number detector according to (1), in which the branching unit is a reflective film, and the photodetection element is a pixel. (3) The photon number detector according to (1) or (2), in which the branching unit is arranged along a surface of the photodetection element where light is input, and the photodetection element detects light transmitted through the branching unit. (4) The photon number detector according to any one of (1) to (3), comprising: a waveguide section that guides light toward the photodetecting element; and an introduction section that introduces light in a photon number state to be measured into the waveguide section, wherein the branching section stochastically transmits light input from the waveguide section and branches the light into the first path and the second path that reflects the light toward the waveguide section. (5) The photon number detector according to (4), wherein the detecting section has a plurality of photodetecting elements, and a plurality of the branching sections are provided corresponding to the plurality of photodetecting elements, and the waveguide section guides the light introduced from the introduction section toward each of the plurality of branching sections. (6) The photon number detector according to (5), wherein the detecting section has the plurality of photodetecting elements arranged along one direction corresponding to the guiding of light by the waveguide section. (7) The photon number detector according to (6), wherein the plurality of branching sections are arranged between the waveguide section and the detecting section. (8) The photon number detector according to (6) or (7), comprising: a first detection unit that is the detection unit arranged on one side in a direction intersecting with the one direction; and a second detection unit that is the detection unit arranged on the other side in the direction intersecting with the one direction. (9) The photon number detector according to (6) or (7), wherein the detection unit is arranged on one side in the direction intersecting with the one direction. (10) The photon number detector according to (9), comprising: a reflecting unit that is arranged on the other side in the direction intersecting with the one direction and reflects light towards the detection unit. (11) The photon number detector according to (9) or (10), wherein the plurality of photodetecting elements are arranged two-dimensionally along a plane.(12) The photon number detector according to (11), further comprising: a light guide section at both ends of the plurality of photodetecting elements in one direction along the surface, which shifts the light in another direction along the surface and reflects it toward the one direction. (13) The photon number detector according to any one of (5) to (12), in which the plurality of branching sections have the same reflectivity. (14) The photon number detector according to any one of (5) to (12), in which at least some of the plurality of branching sections have different reflectivities. (15) The photon number detector according to (14), in which each of the plurality of branching sections has a reflectivity equal to or lower than the reflectivity of the branching section that reflected input light. (16) The photon number detector according to (14) or (15), in which each of the plurality of branching sections has a reflectivity that attenuates with increasing distance from the introduction section. (17) The photon number detector according to any one of (4) to (16), further comprising: a photon branching unit that branches light before introduction into the waveguide unit; a plurality of the waveguide units are provided corresponding to paths branched by the photon branching unit; and a plurality of the detection units are provided corresponding to the plurality of waveguide units.

[0205] 1, 1A to 1F Photon number detector (detection system) 2, 2A Introduction section 3, 30 Waveguide section 4 Branching section 5, 50 Detection section 6 Beam splitter 7 Wavelength-dependent beam splitter 8 Optical switch 9, 90 Reflection section 10 Photodetection element 20, 21 Detection unit 41, 42 Light guide section

Claims

1. A photon number detector comprising: a detection unit having at least one photodetection element capable of single photon detection; and a branching unit that probabilistically transmits input light and branches it into a first path toward the photodetection element and a second path different from the first path.

2. The photon number detector according to claim 1, wherein the branching portion is a reflective film, and the light detection element is a pixel.

3. The photon number detector according to claim 1, wherein the branching portion is disposed along a surface of the photodetecting element to which light is input, and the photodetecting element detects light transmitted through the branching portion.

4. A photon number detector as claimed in claim 1, comprising: a waveguide section which guides light towards said photodetector element; and an introduction section which introduces light in a photon number state to be measured into said waveguide section, wherein said branching section stochastically transmits light input from said waveguide section and branches the light into said first path and said second path which reflects the light towards said waveguide section.

5. A photon number detector as claimed in claim 4, wherein the detection section has a plurality of photodetection elements, the branching section is provided in a plurality corresponding to the plurality of photodetection elements, and the waveguide section guides the light introduced from the introduction section towards each of the plurality of branching sections.

6. The photon number detector according to claim 5, wherein the detection section has the plurality of photodetection elements arranged along one direction corresponding to the guiding of light by the waveguide section.

7. The photon number detector according to claim 6, wherein the plurality of branching sections are disposed between the waveguide section and the detection section.

8. A photon number detector as described in claim 6, comprising: a first detection unit which is the detection unit arranged on one side in a direction intersecting with said one direction; and a second detection unit which is the detection unit arranged on the other side in a direction intersecting with said one direction.

9. The photon number detector according to claim 6, wherein the detection unit is disposed on one side in a direction intersecting the one direction.

10. The photon number detector according to claim 9, further comprising: a reflecting section arranged on the other side in a direction intersecting the one direction, for reflecting light towards the detecting section.

11. The photon number detector according to claim 9, wherein the plurality of photodetection elements are arranged two-dimensionally along a surface.

12. The photon number detector according to claim 11, further comprising a light guiding section at both ends of the plurality of photodetection elements in one direction along the surface, which shifts the light in another direction along the surface and reflects it toward the one direction.

13. The photon number detector according to claim 5, wherein the plurality of branching sections have the same reflectance.

14. The photon number detector according to claim 5, wherein at least some of the multiple branches have different reflectivities.

15. The photon number detector according to claim 14, wherein each of the plurality of branching sections has a reflectance equal to or lower than the reflectance of the branching section that reflected the input light.

16. The photon number detector according to claim 14, wherein each of the plurality of branch sections has a reflectance that attenuates with increasing distance from the introduction section.

17. The photon number detector according to claim 4, further comprising: a photon branching section that branches light before introduction into the waveguide section, wherein a plurality of the waveguide sections are provided corresponding to paths branched by the photon branching section, and a plurality of the detection sections are provided corresponding to the plurality of waveguide sections.

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